Throughout history, people with intellectual disabilities have been viewed in a number of negative ways. They have been classified as “less than human” and “burdens upon society,” and their conditions have been thought to be punishments from God and the effects of demonic possession. As a result of these negative historic views, people with intellectual disabilities have endured cruel treatment. Ancient Greeks and Romans killed infants that they believed to have intellectual disabilities. In the Middle Ages people with intellectual disabilities were locked in asylums, and tens of thousands were killed in Nazi Germany in the late 1930s and early 1940s as part of Adolf Hitler’s attempt to create a “superior” human race.

Fortunately, such treatment of people with intellectual disabilities (people who have below-average intellectual function and limitations in areas necessary for normal, daily living) is not tolerated in modern society. The United States government has adopted several laws to improve the rights of people with intellectual disabilities. For example, the Americans With Disabilities Act of 1990 gives civil rights to people with disabilities, guaranteeing equal opportunities for these individuals in employment, public accommodations (e.g., restaurants, theaters), transportation (e.g., public bus and train systems), and government services (facilities and communications must be accessible to people with disabilities).

Perceptions of people with intellectual disabilities have changed over time as well. For example, people are now more sensitive toward the feelings of those with intellectual disabilities and have moved away from describing these individuals as “mentally retarded” because that term has become loaded with subtext and is frequently hurtful and offensive. Additionally, while doctors cannot find specific reasons for people’s intellectual disabilities in many cases, they now know that causes for intellectual disabilities can be biological, physical, or environmental. Finally, treatment goals now include trying to help people with intellectual disabilities reach their highest level of functioning. Although the outlook for a person with intellectual disabilities depends on his or her level of impairment, as a result of these changing perceptions and treatment goals, many people with intellectual disabilities can now lead independent, productive lives.

Intellectual Disability: An Overview

People with intellectual disabilities make up between one and three percent of the United States population. Intellectual disabilities originate before a person is 18 years old and are characterized by below-average intellectual functioning (e.g., trouble with reasoning, learning, and problem solving) and limitations in social and practical skills that are necessary for daily life. Limitations in the skills that are necessary for normal, daily living can impair people’s abilities to communicate, take care of themselves (e.g.make their own decisions, dress themselves, and feed themselves), interact and participate in social situations, and look out for their personal health and safety, for instance.

Although people with intellectual disabilities develop slowly and with more difficulty than people who do not have intellectual disabilities, having intellectual disabilities does not mean that a person cannot learn. As with other medical conditions, there are varying degrees of severity with intellectual disabilities. Some people are only mildly limited and can learn communication, social skills, and practical skills. Others are severely limited and may never be able to function independently.

Intellectual disabilities used to be referred to as “mental retardation,” but a negative social stigma has become associated with the term “mental retardation.” Today, the term is viewed as hurtful, offensive, and emotionally damaging, so people have replaced it with the phrase “intellectual disability.”

People may also use the terms “developmental disorder” or “developmental disability” interchangeably with the term “intellectual disability,” but these are not exactly equivalent. ”Developmental disability” and “developmental disorder” are umbrella terms that encompass a wide range of disabilities including physical and intellectual disabilities. Intellectual disability is a type of developmental disability, but some developmental disabilities are purely physical (e.g., blindness from birth) and do not have an intellectual component.

Causes and Risk Factors of Intellectual Disability

Many times, doctors cannot identify a specific cause for a person’s intellectual disabilities. However, it is known that intellectual disabilities can occur when the brain is prevented from growing and developing normally.

Causes of intellectual disability can include:

• Causes that Occur before Birth

• Certain Inherited Disorders: These diseases are passed from parents to their children through genes. A fetus’s genetic make up is determined when it is conceived, and some genetic disorders can prevent the fetus’s brain from growing and developing properly.

Tay-Sachs disease is one such disorder. People with Tay-Sachs disease cannot properly metabolize a lipid (or fatty substance) called GM2 ganglioside. When this lipid builds up, it causes damage to the cells of the brain.

Phenylketonuria (PKU) is another such disorder. People with PKU cannot properly break down a substance called phenylalanine that is found in many foods. When the level of phenylalanine gets too high, it causes brain damage.

Other examples of inherited disorders that can cause growth and development problems in the brain include neurofibromatosis, hypothyroidism, Hunter syndrome, and Rett syndrome.

• Chromosomal Abnormalities: Each cell in the human body contains 23 pairs of chromosomes (for a total of 46), and each chromosome contains thousands and thousands of genes. When there is a problem with a chromosome, it affects the instructions for how the body of a fetus is supposed to develop. Mistakes or abnormalities in certain chromosomes can affect the development of the fetal brain.

For example, in cri du chat syndrome, part of chromosome number five is missing, including a gene that helps to control cell growth. Down syndrome is another example of a chromosomal abnormality that can lead to intellectual disability. People with Down syndrome have an extra copy of chromosome number 21, which leads to mental and physical developmental delays.

Other chromosomal abnormalities that can lead to brain growth and developmental problems include chromosomal translocations (when a gene is located in an unusual place on a chromosome or is located on a different chromosome than it should be), fragile X syndrome, Angelman syndrome, and Prader-Willi syndrome.

• Severe Maternal Malnutrition: When a mother does not get enough nourishment, it can affect the growth and development of the fetus’s brain.

 

• Infections: If a mother becomes infected with certain diseases when she is pregnant, those infections can harm the development of her fetus as well. These infections are particularly dangerous during a mother’s first three months of pregnancy when the fetal brain begins to form. Examples of potentially harmful infections include HIV, herpes simplex, toxoplasmosis, cytomegalovirus, and rubella.

 

• Exposure to Drugs or Toxins: When a mother consumes drugs or is exposed to other toxins, often her fetus is exposed to these substances as well. Substances that can interfere with the development and growth of the fetal brain include alcohol, lead, cocaine, amphetamines, and chemotherapy drugs to treat cancer.

• Causes that Occur during Birth

• Extreme Prematurity: Babies who are born less than 34 weeks after they were conceived have a greater chance for intellectual disability. One reason for this is because these babies have an increased risk of bleeding in their brains during their deliveries. During labor, changes occur in a baby’s circulation, and when a baby is very premature, his or her immature blood vessels may rupture because they cannot handle these changes. Bleeding in the brain can cause damage that prevents the baby’s brain from continuing to grow and develop normally.

• Causes that Occur after Birth

• Brain Infections: A baby or child can get infections that affect the continued growth and development of the brain. For example, meningitis is an infection of the membranes covering the brain and spinal cord (called meninges) and can cause brain damage. Encephalitis is irritation and swelling (or inflammation) of the brain that is often caused by viral infections. It can lead to nerve cell destruction and bleeding in the brain, which can cause brain damage as well.

 

• Severe Head Injuries: Severe head injuries can cause bleeding in the brain, which often results in brain damage. Head injuries may also involve direct force to the brain that is strong enough to cause permanent damage.

 

• Malnutrition: The brain needs certain nutrients to function properly (e.g., iodine). When children do not get these nutrients, the development of their brains does not progress normally.

 

• Severe Emotional Neglect or Abuse: In order for a baby’s or a child’s brain to develop normally, it needs daily stimulation. If a baby or child is neglected or emotionally abused and thus does not receive this stimulation, his or her brain will not develop properly.

 

• Exposure to Toxins: Certain substances, such as lead and methylmercury are poisonous. Lead can be found in toys, older house paint, dust, and dirt, and methylmercury can be consumed when people eat animals fed seed, grain, or fish from waters contaminated with methylmercury. When ingested by babies and young children these substances can cause brain damage.

 

• Brain Tumors: Brain tumors (masses or growths of abnormal cells) can damage brain tissue directly and can also increase cranial pressure, which can lead to brain damage as well.

• Causes that Can Occur before, during, or after Birth.

• Insufficient Oxygen to the Brain: The brain needs a constant supply of oxygen to function properly. When the brain does not get enough oxygen, the brain cells start to die, resulting in brain damage. A lack of oxygen to the brain is a risk to occur before a baby is born, during the birth, and after the baby is born as well.

Symptoms of Intellectual Disability

Symptoms of intellectual disability depend on the severity of the disability. Symptoms of severe disability may be apparent as soon as a child is born (e.g., physical abnormalities). In a case of milder disability, symptoms may not become apparent until the child enters school.

Symptoms of intellectual disability can include the following:

• Continued infant-like behavior (this is associated with severe intellectual disability)

 

• Difficulty learning and a decreased ability to learn

 

• Failure to reach milestones of intellectual development (e.g.,rolling over, walking, talking) or reaching these milestones much later than peers

 

• Motor, language, or self-help skills that do not seem to be developing or are developing at a much slower rate than those of a child’s peers (e.g., child’s abilities to roll over, sit, stand, use words, speak in complete sentences, feed and dress him or herself, etc)

 

• Inability to meet the educational demands of school

 

• A lack of curiosity

 

• Failure to adapt and adjust to new situations

 

• Physical abnormalities (e.g.,unusual facial features, a head that is too large or too small, deformities of the hands or feet, etc)

Diagnosing Intellectual Disability

Diagnosing intellectual disabilities involves trying to determine the cause of the disabilities and then evaluating a child’s intellectual functioning and age-appropriate adaptive skills. Doctors and other health professionals (e.g., psychologists, speech pathologists, occupational therapists, physical therapists, social works, and nurses) look for the causes of children’s intellectual disabilities for several reasons:

• First, although many of the causes of intellectual disabilities are irreversible, such as chromosomal abnormalities, others can be treated such that children develop normally. For example, hypothyroidism (or underactive thyroid) is a condition where the thyroid gland does not produce enough of certain hormones. This can lead to problems with mental development. However, if this condition is caught in a baby within the first few months of birth, it can be treated, and the chances for normal development are good.

 

• Additionally, conditions other than intellectual disability can cause children to have developmental delays. For instance, hearing problems can interfere with language and social development, and delays in sitting or walking can be related to nerve problems rather than to intellectual disabilities.

 

• Finally, trying to identify the cause of a child’s suspected intellectual disability may help doctors predict the child’s future developmental course, prevent further delays in social and behavioral skills, plan a treatment program that can increase the child’s level of functioning, and inform the child’s parents about the risks of having another child with an intellectual disability.

Doctors and other health professionals may look for the causes of a suspected intellectual disability by performing a physical exam, collecting a child’s medical history, and then potentially conducting laboratory tests, imaging tests like magnetic resonance imaging (MRI) to look at the brain for structural problems, chromosome analyses, urine and blood tests, and X-rays.

Depending on the results of these physical tests, doctors may then give the child a series of standardized intelligence tests, which are designed to determine his or her intelligence quotient (or IQ). A score below 70 on a standardized IQ test can indicate that a child has intellectual disabilities. Doctors and other health professionals may also give the child more standardized tests to measure communication skills, daily living skills, social abilities, and motor skills. Because these tests are standardized, they allow doctors and other health professionals to compare a child’s score to scores of other children who are around the same age. A child may be diagnosed as having an intellectual disability when his or her results on both the intellectual tests and the adaptive skills tests are significantly below average.

However, these standardized tests are not perfectly unbiased, and some children may have a more difficult time on them because they have different cultural backgrounds, do not speak English, or are at a very low socioeconomic status. So, doctors and other health professionals also use direct observations of a child and interviews with his or her parents to diagnose intellectual disabilities.

Treatment and Outlook

Treating a person with an intellectual disability usually involves a collaborative effort from a team of professionals, including doctors, social workers, speech therapists, occupational therapists, physical therapists, psychologists, educators, etc. Along with the person’s family members, this team develops a comprehensive, individualized treatment program with the goal of helping the person with intellectual disabilities develop to his or her fullest potential. Family members may find it difficult to care for a person with intellectual difficulties, so it is important that the treatment plan include support for them in the form of counseling, day care centers, social work, etc.

The treatment program should begin as soon as an intellectual disability is suspected (even in infancy) and should be tailored to the person’s strengths and weaknesses with regard to communication, social skills, practical skills, etc. The program will likely include behavioral approaches to teach the person the social skills and adaptive skills necessary for normal, daily life. Additionally, the program should include special education for school-aged children, which public schools in the United States are required by law to provide. This education is required to be in the most-inclusive setting, where children have the opportunity to interact with peers without intellectual disabilities whenever possible.

The outlook for people with intellectual disabilities depends on several factors, including how severe their disabilities are, whether or not they have other health conditions (e.g., brain tumors), and what types of opportunities and treatment they receive. People with more severe intellectual disabilities will likely need assistance with even the most basic tasks throughout their lives. However, many people with mild to moderate intellectual disabilities can live independently, support themselves, and be successful in jobs that require basic intellectual skills.